The present invention relates to an image forming apparatus and, more particularly, to an electrophotographic copier, laser printer, facsimile transceiver or similar image forming apparatus of the type having a polygonal mirror.
There is an increasing demand for an image forming apparatus capable of forming an image at high density and in multiple tones. In this respect, banding which occurs in the subscanning direction of an image and lowers the image quality is an obstacle. One of major causes of banding is the irregularity in the position where a laser beam is incident to a polygonal mirror. Specifically, when the individual mirror surfaces of a polygonal mirror are different in angle, the position where a laser beam is incident to the mirror differs from one surface to another. Since a developing unit and a fixing unit as well as the polygon mirror are driven by motors and, therefore, necessarily causes mechanical vibrations, the vibrations are transmitted to a laser unit and mirrors to change the position where the laser beam is incident to a photoconductive element, or image carrier, again resulting in banding. Further, when the moving speed of the photoconductive element changes, the subscanning pitch is changed to cause banding to occur in an image. The change in the moving speed of the photoconductive element is ascribable to the irregularity in the rotation speed of a motor which drives the element, the eccentricity of a shaft on which the photoconductive element implemented as a drum is mounted, the eccentricity of a roller for driving the photoconductive element implemented as a belt, the eccentricity of gears and pulleys which constitute gearing, the change in speed due to backlash, etc. In the light of this, it has been customary to drive the photoconductive element by an expensive motor whose rotation speed changes little, accurate gearing, or a flywheel having a great moment of inertia.
Regarding the banding ascribable to the configuration of the polygonal mirror, Japanese Patent Laid-Open Publication No. 91211/1981 proposes an anti-banding system in which a beam is emitted onto a mirror surface of a polygon mirror other than the mirror surface to which a data beam is incident. A signal representative of the inclination angle of the particular mirror surface is generated beforehand in response to a reflection from the mirror surface. When the particular mirror surface arrives at a position where the data beam should be incident, the incidence angle of the data beam to the polygon mirror is controlled on the basis of the above-mentioned signal.
Various approaches for eliminating banding due to the irregular rotation of the photoconductive element are shown and described in Japanese Utility Model Laid-Open Publication No. 193518/1985 and Japanese Patent Laid-Open Publication Nos. 7262/1987, 65275/1986, and 11011/1986l Laid-Open Publication No. 193518/1985 teaches a system which displaces a laser or a collimating lens mechanically by an actuator in a direction parallel to the axis of a polygonal mirror and in synchronism with the rotation of the mirror. Laid-Open Publication No. 7262/1987 discloses a system which senses the angular position of a photoconductive element and, based on the resultant signal, sets an exposure start timing and an exposure end timing of exposing means. Laid-Open No. 65275/1986 proposes an arrangement in which an optical writing device writes a light image on a photoconductive element in synchronism with the transport speed of a photoconductive element. Further, Laid-Open Publication No. 110115/1986 teaches a system which drives, when detected a laser beam deflected by a polygonal mirror, a photoconductive element by a predetermined angle which is dependent on the the intervals of scanning lines.
The problem with Laid-Open Publication No. 91211/1981 is that the structure is complicated and, moreover, the fluctuation in the rotation speed of the photoconductive element is not considered at all. Laid-Open Publication No. 193518/1985 has a drawback that the mechanical displacement relying on an actuator cannot respond to speed ripples rapidly and, therefore, fails to correct high-speed fluctuation. Laid-Open Publication Nos. 7262/1987 and 65275/1986 both are applicable only to an LED array and an LCD array and need complicated circuitry. Laid-Open Publication No. 110115/1986 cannot eliminate the fluctuation in rotation since it implements the control over the rotation of the photoconductive element by drive transmission using a timing belt and gears.